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Beam Induced Background in ALICE

Beam Induced Background in ALICE. Michele Floris LBS Meeting 19/04/2010. Outline. Triggering Detectors in ALICE Background Estimates Background Reduction Cuts Comparison to Simulation @ 900 GeV 7 TeV: a first look Conclusions and Outlook. NB: Offline perspective in this talk

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Beam Induced Background in ALICE

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  1. Beam Induced Background in ALICE Michele Floris LBS Meeting 19/04/2010

  2. Outline • Triggering Detectors in ALICE • Background Estimates • Background Reduction Cuts • Comparison to Simulation @ 900 GeV • 7 TeV: a first look • Conclusions and Outlook NB: Offline perspective in this talk Mainly 900 GeV data LBS Meeting

  3. Minimum Bias Trigger, 900 GeV h Silicon Pixel Detector 9.8 M Channels “Fast OR” signal 2 layers, |h| < 2.0/1.4 Tracklets: 2 points + vertex V0s Scintillator slabs Time readout (allows for BG rejection) 2.8 < h < 5.1 and -1.7 < h < -3.7 V0A V0C OR OR SPD Fast Or (FO) LBS Meeting

  4. Trigger suite • Minimum Bias Trigger (SPD FO or V0A or V0C) in coincidence with signals from beam pickups • Trigger on: • Bunch-Bunch (CINT1B) • Collisions + Beam-Background + Noise • Bunch-Empty (CINT1A/CINT1C) • Beam-Background + Noise • Empty-Empty (CINT1E) • Noise (i.e. Accidentals) LBS Meeting

  5. Background Estimates Background can be estimated from control triggers as: If intensities in non-interacting and interacting bunches differ, one needs to reweight to get the BG in the CINT1B: • Background reduction cuts based on: • Timing distribution in the V0 detector • Correlation cluster vs tracklet in the SPD LBS Meeting

  6. Bunch Intensities • DIP did not publish intensities for the whole period • Data from Timber (available as CSV file) • One column/bunch for each beam Issues: • Lots of afterpulses in the early fills • Some measurements for Beam2 in the wrong column (constant shift) • At low intensity: unreliable measurements? • Lots of missing data points • Inter(Extra)polate to get mean • Outliers (remove point) NB: we only used ratios of intensities for this study LBS Meeting

  7. V0 Time Distributions V0A V0 time distributions for the different trigger classes Used to reject beam BG events IP V0C V0A LBS Meeting

  8. V0 Time Windows and BG Cuts V0A V0C Collisions Collisions Beam Background Beam Background Offline selection: (V0A || V0C || SPD) && !V0ABG && !V0CBG LBS Meeting

  9. Cluster vs Tracklets Correlation between tracklets and clusters in the SPD: You need more cluster to make a tracklet if the event is not produced in the interaction region Tracklet := track segment made out of 2 points in the 2 SPD layers CINT1B CINT1A+CINT1C LBS Meeting

  10. BG vs Run Number Without Cuts With Cuts Background ~ 2-6% relative to CINT1B Rate of triggered BG events ~ 1 Hz (no dead time correction) Cuts reduce BG significantly  Under control for physics analysis, residual BG low multiplicity Within a fill, BG increases with time: size of luminous region Correlation with machine parameters to be studied LBS Meeting

  11. Beam Gas / Halo Simulations • Beam Gas |z| < 20 m • pC collisions (HIJING), assuming a constant pressure profile • Beam Halo • Beam gas interactions outside experimental region • Interaction products deflected/transported by magnets • Tertiary/Quartiary Halo, cleaning inefficiency Interactions on tertiary collimators • Simulations use the boundary source provided by Rob  Appleby LBS Meeting

  12. Comparison Data-Simulation NORMALIZED TO THE INTEGRAL: COMPARING ONLY THE SHAPE! Agreement data – beam halo simulation: consistent with the expectation that most of the BG comes from halo No Event Selection (Kills BG events w/ tracks) TPC tracks, loose cuts (No DCA cuts) LBS Meeting

  13. Comparison to simulation – 2 Simulation (Gas+Halo) Data (CINT1A) Beam GAS and Beam are very similar. Consistent with data LBS Meeting

  14. Background at 7 TeV: a first look CINT1B CINT1A+CINT1C Similar behavior to 900 GeV BG/CINT1B = 0.3 – 0.6 % LBS Meeting

  15. Conclusions and Outlook • Beam Induced Background is under control for physics analysis (with current situation) • After cuts, very low residual background (low multiplicity) • Can be treated with control triggers • Background studies: Next Steps • Absolute comparison data/MC • Radial hits distribution using FMD • Correlation with intensity and machine data • Detailed studies at 7 TeV LBS Meeting

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